1. Field of Invention
The present invention relates in general to control systems of electric motors and more particularly to control systems of a type that inverts direct current from a battery to alternating current to power an electric motor. More specifically, the present invention is concerned with a technique that effectively uses a direct current power source for powering an electric motor.
2. Description of Related Art
Hitherto, various control systems of electric motors have been proposed and put into practical use. One of them is described in Laid-Open Japanese Patent Application (Tokkai) 2000-32799. In the control system of this publication, for various different voltages from a battery, there are provided respective current command maps listing high-efficiency data in advance by torque commands and rotation speeds of the motor, and according to the voltage actually applied to the motor, an optimum one of the current command maps is selected. With this, even if a battery voltage is subjected to a certain fluctuation, a maximum terminal voltage is available to the motor and thus, the battery power is effectively used.
In order to clarify the task of the present invention, the control system disclosed by the above-mentioned publication will be briefly discussed in the following.
That is, under a normal condition of the control system, an optimum current command map is selected in accordance with the battery voltage to derive a current command. However, in case of a marked change of the motor torque, that is, in case wherein fluctuation of the current command becomes marked, the battery voltage becomes varied temporarily due to output fluctuation of the motor, so that the voltage is subjected to a voltage drop prior to switching of the current command maps. In this case, the control system tends to lower the control accuracy. If, for suppressing lowering of control accuracy, margins are provided to the current command maps, working efficiency of the motor becomes lowered inevitably.
If a feedback cycle is made shorter, that is, if the interval at which torque command and rotation speed data of the motor are issued is made shorter, the fluctuation of battery voltage can be easily tracked or controlled. However, this measure needs addition of a noise cut filter for removing noises which are inevitably produced when monitoring the battery voltage, and thus the measure brings about lowering in tracking ability. In addition to this, the load of the control system becomes larger.
Accordingly, an object of the present invention is to provide a control system of an electric motor, which is free of the above-mentioned defects.
According to a first aspect of the present invention, there is provided a control system of an electric motor powered by a battery, which comprises an inverter circuit that inverts a direct current fed from the battery to an alternating current fed to the electric motor; and an output controlling circuit that controls an output of the inverter circuit to adjust operation of the electric motor, the output controlling circuit including a first section that detects a magnitude of output voltage of the battery; a second section that, based on the detected output voltage, derives an unloaded voltage that would be outputted from the battery when the battery is unloaded; a third section that, based on the unloaded voltage, looks up an appropriate current command from maps that store various current commands with respect to operation condition of the motor; and a fourth section that, based on the looked up current command, controls the output of the inverter circuit.
According to a second aspect of the present invention, there is provided a control system of an electric motor powered by a battery, which comprises an inverter circuit that inverts a direct current fed from the battery to an alternating current fed to the electric motor; and an output controlling circuit that controls an output of the inverter circuit to adjust operation of the electric motor, the output controlling circuit including a voltage detecting section that detects a magnitude of an output voltage of the battery; an unloaded voltage estimation section that, based on the detected output voltage, estimates an unloaded voltage that would be outputted from the battery when the battery is unloaded; a map storing section that stores a plurality of maps for various unloaded voltages, each map showing a relationship between a torque command, a rotation speed of the motor and a current command, the map storing section issuing a corresponding current command when receiving the unloaded voltage from the unloaded voltage estimation section; and a current control section that controls the output of the inverter circuit in accordance with the appropriate current command from the map storing section.
According to a third aspect of the present invention, there is provided a control system of an electric motor powered by a battery, which comprises an inverter circuit that inverts a direct current fed from the battery to an alternating current fed to the electric motor; and an output controlling circuit that controls an output of the inverter circuit to adjust operation of the electric motor, the output controlling circuit including a voltage detecting section that detects a magnitude of an output voltage of the battery; an unloaded voltage estimation section that, based on the detected output voltage, estimates an unloaded voltage that would be outputted from the battery when the battery is unloaded; a position sensor that detects an angular position and a rotation speed of a rotor of the motor; a current sensor that detects a condition of current outputted from the inverter circuit; a host controller that issues a torque command; a map storing section that stores a plurality of maps for various unloaded voltages, each map showing a relationship between the torque command, the rotation speed of the electric motor and a current command, the map storing section issuing a corresponding current command when receiving the unloaded voltage from the unloaded voltage estimation section; and a current control section that controls the output of the inverter circuit in accordance with the corresponding current command from the map storing section, the detected angular position and rotation speed of the rotor of the electric motor from the position sensor and the detected condition of current from the current sensor.